1. The Field of the Invention
The present invention relates to vertical cavity surface emitting lasers. More particularly, the present invention relates to vertical cavity surface emitting lasers that use photonic crystals to achieve high energy cavities.
2. Background and Relevant Art
Some of the most commonly used light sources in optical communication systems are semiconductor lasers. Vertical cavity surface emitting lasers (VCSELs) are an example of semiconductor lasers and are used in optical communication systems for several reasons. VCSELs can be manufactured in large quantities due to their relatively small size and can often be tested on a single wafer. VCSELs typically have low threshold currents and can be modulated at high speeds. VCSELs also couple well with optical fibers.
VCSELs are typically made from GaAs semiconductor materials as opposed to InP materials because GaAs semiconductor materials make better multi-layer mirror systems than InP materials. A VCSEL typically requires a high reflectivity mirror system because in a VCSEL, the light resonates in a direction that is perpendicular to the pn-junction. The cavity or active region of a VCSEL is thus relatively short and a photon has little chance of stimulating the emission of an additional photon with a single pass through the active region. To increase the likelihood of stimulating the emission of photons, VCSELs require highly efficient mirror systems such that a photon can make multiple passes through the active region. The reflectivity requirement of VCSELs cannot be satisfied or achieved with metallic mirrors.
VCSELs thus employ Distributed Bragg Reflector (DBR) layers as mirrors. DBR layers are formed or grown using, for example, semiconductor or dielectric materials. DBR layers are grown or formed such that each layer has a refractive index that is different from the refractive index of adjoining layers. The junctions between the DBR layers that are grown in this fashion cause light to be reflected. The amount of light reflected, however, by a single junction is relatively small and dependent on the variance between the relative refractive indices of the adjoining materials. For this reason, a relatively large number of DBR layers are formed in a VCSEL in order to achieve high reflectivity. VCSELs, for example, often have on the order of 50 to 100 DBR layers in order to achieve sufficient reflectivity. The large number of DBR layers also increases the resistance of the VCSEL and may lead to problems with both heat during operation and the growth or formation of the layers.
Another problem associated with VCSELs is related to the wavelength of the light that is generated. Most VCSELs generate light that has a wavelength of approximately 0.85 microns. This wavelength is primarily useful in very short distance fiber optic communications but is typically inadequate for longer distance fiber optic networks such as telecommunication networks. Attempts to develop and fabricate VCSELs that operate at longer wavelengths (1.3 microns and 1.55 microns, for example) have proven to be very difficult. This difficulty is related to the fact that InP materials permit the growth of a suitable active region, but the DBR layers are not effective. When GaAs materials are used, the growth of the DBR layers is straightforward, but the active region is unsuitable. These attempts have resulted VCSELs that produce insufficient power or are unreliable.
VCSELs that generate light at longer wavelengths such as 1.3 and 1.55 microns, which are useful in longer distance optical communication networks, are difficult to fabricate. Some of the reasons that VCSELs that emit longer wavelengths are difficult to fabricate and design include the requirement of successfully designing and forming many DBR layers, the need for the lattice structures of the various layers in the VCSEL to match, and the complexity of growing the DBR layers successfully.
These and other limitations are overcome by the present invention, which relates to vertical cavity surface emitting lasers and to methods of fabricating vertical cavity surface emitting lasers. The present invention forms at least one of the mirror layers of a VCSEL using photonic crystals or a combination of photonic crystals and Distributed Bragg Reflector (DBR) layers.
A photonic crystal or layer is a material, such as a semiconductor material or a dielectric material, that has cavities or apertures or other structure that is formed in the material. The photonic crystal is typically formed such that the cavity structure is periodic. The periodic cavity structure of a photonic crystal is similar to the atomic lattice structure of various materials, but on a larger scale. The periodic cavity structure of a photonic crystal is not confined to the atomic lattice structure of the material and can be formed as required using various configurations and shapes. Because the photonic crystals used in the present invention are planar in nature, the periodic cavity structure is usually two dimensional, although a three dimensional photonic crystal is contemplated by the present invention.
In one example, an active region is formed between photonic crystals. The photonic crystals provide the necessary reflectivity such that photons are reflected between the photonic crystals through the active region, which results in stimulated emission of photons at the corresponding wavelength of the incident photon. The photonic crystals can be combined with DBR layers in other embodiments to provide alternative mirror systems. With a photonic crystal or layer, however, the number of DBR layers is typically reduced.
The VCSEL can be tuned to produce different wavelengths by varying attributes or characteristics of the photonic crystal. Exemplary attribute changes include, but are not limited to, changing the cavity structure to another configuration, altering the shape of the individual cavities, adding another photonic crystal, and the like or any combination thereof. With photonic crystals as mirrors, longer wavelengths can be generated by the VCSEL. The VCSEL can be designed to emit different wavelengths, for example, by using various combinations of cavity shape, cavity structure, cavity orientation, photonic crystal material, and the like. The VCSEL can also be configured to emit a particular wavelength by controlling the refractive index of the photonic crystal by filling the cavities with some material. Additional layers of photonic crystals may extend the band of wavelengths for which high reflectivity is achieved.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.